The present invention relates generally to a contact type image sensor, and, more particularly, to a contact type color image sensor formed using amorphous semiconductor film.
In general, when the document image to be transmitted is read in a facsimile system, that optical image is focused onto the photoelectric converting unit. This photoelectric converting unit is formed of a small solid-state CCD imaging device of 2048 bits and approximately 32 to 46 mm long. A lens is used to focus the image of the document on the photosensitive surface of this small CCD image sensor and to reduce the image size. A typical A4 document 200 mm wide must be reduced to 32-46 mm. In order to obtain this degree of reduction the distance from the document to the image sensor must be increased, which makes it difficult to build a compact facsimile terminal.
In consideration of this problem, contact type image sensors have been developed. With this kind of sensor, the length of the sensor array is substantially the same as the width of the document, which means that image reduction before the image reaches the image sensor is not necessary. With this kind of construction, the length of the optical path from the document surface to the photoelectric converting unit can be decreased to around 10 mm because a real-size (1:1 magnification) optical system, such as a rod lens array is used for forming the image.
The demand for a color facsimile system has increased the need for development in contact type linear image sensors. One problem is that if the sensor is made to provide color images, the signal-to-noise ratio (S/N ratio) of the image reading will decrease. The reason for this decrease is that color separation of the input optical image is carried out in the lengthwise direction of the linear CCD image sensor, i.e., the main scanning direction. Consequently, the number of picture elements (pixels) in the main scanning direction is reduced to the equivalent of one third.
In order to solve this problem, the inventors have proposed arranging a plurality of cells for sensing the three primary colors of one pixel in the width direction of the contact type image sensor (Japanese Patent Application No. 58-220972). With this improvement in the arrangement of the cells, the color separation is carried out in the sub-scanning direction of the contact type image sensor, so it is possible to prevent a reduction in the S/N ratio. The result of this is that the color image sensor is improved to have the same S/N ratio as a monochrome sensor to which a color filter has been attached.
In the embodiment of the above concept, when the pixel electrodes of the contact type image sensor are arranged contiguously along the sub-scanning direction, they must be made smaller than the pixel electrodes in the main scanning direction. Consequently, the photosensitive area is reduced by the amount of the decrease in size of the electrodes in the sub-scanning direction. This decrease in photosensitive area brings with it (1) a lower S/N ratio and (2) great difficulty in positioning of the sensor in relation to the pixels of the color filter, making mounting on the sensor difficult.